1. Field of the Invention
The present invention relates to a transmission device and transmission method for amplifying and transmitting an analog signal that has been converted from a digital input signal, and in particular relates to a transmission device and transmission method for predistorting a digital signal in order to compensate distortion, then amplifying and transmitting it.
2. Description of the Related Art
As out-of-band power emission in wireless communications systems is regulated, such systems are equipped with linearizers (distortion compensating devices) in order to minimize out-of-band power emission into surrounding channels due to non-linearity of the amplifiers that amplify signals for transmission. FIG. 13 is a block diagram of an exemplary transmission device in a conventional wireless communications system.
This transmission device has a predistortion linearizer that includes a multiplier 210, a power calculator 211, a compensation table 212, a distortion-compensation coefficient update unit 213, and a subtracter 214.
Each of four baseband transmission signals (each signal composed of a digital I signal and a digital Q signal) is modulated by one of four modulators 201 to 204 and then subjected to frequency shifting by frequency shift multipliers 205 to 208 so as to avoid overlapping frequency bands. The four transmission signals are then converted into a single digital signal (one digital I signal and one digital Q signal) by an adder 209, and supplied to the multiplier 210, power calculator 211, and subtracter 214.
The power calculator 211 calculates a power value p for an input transmission signal and supplies the power value p to the compensation table 212. The compensation table 212 then supplies a distortion-compensation coefficient corresponding to the power value p to the multiplier 210 and the distortion-compensation coefficient update unit 213. The input transmission signal and distortion-compensation coefficient are multiplied by the multiplier 210 to effect predistortion of the transmission signal.
In a modulating NCO (numerically controlled oscillator) 215 the transmission signal output by the multiplier 210 is subjected to quadrature modulation to a single digital signal and is converted to an intermediate frequency band signal which is supplied to a DAC (digital/analog converter) 216.
The input digital signal is converted into an analog transmission signal by the DAC 216. The analog signal is passed through a filter 217 to eliminate the baseband components, and is then converted to radio frequency band by an RF mixer 218, amplified by an amplifier 219, and transmitted from an antenna 220.
A portion of the output signal from the amplifier 219 is supplied as a feedback signal to an IF mixer 221.
At the IF mixer 221 the feedback signal is converted from the radio frequency band to the intermediate frequency band and then attenuated by a variable attenuator 222 by the inverse of the amplification factor of the amplifier 219. The attenuated feedback signal is eliminated of the radio frequency band components by a filter 223 and is then converted to a digital signal by an ADC (analog/digital converter) 224.
This digital feedback signal is subjected to quadrature demodulation by a demodulating NCO 225, converted from the IF band to baseband, and supplied to the subtracter 214 and distortion-compensation coefficient update unit 213.
The feedback signal supplied to the subtracter 214 is subtracted from a reference signal from the adder 209 to determine a differential signal for the two signals. This differential signal is equivalent to a distortion signal component contained in the feedback signal. This distortion signal component is supplied to the distortion-compensation coefficient update unit 213.
On the basis of the distortion signal component from the subtracter 214, the distortion-compensation coefficient from the compensation table 212, and the feedback signal from the demodulating NCO 225, the distortion-compensation coefficient update unit 213 determines a new distortion-compensation coefficient, whereupon the compensation table 212 is updated with this new distortion-compensation coefficient. The new distortion-compensation coefficient is used to compensate a subsequently input transmission signal.
The above procedure is repeated for each input transmission signal.
Thus, in conventional transmission devices, distortion-compensated signals are converted from digital signals to analog signals by a DAC 216. However, as distortion-compensated signals typically have greater amplitude than do the same signals prior to compensation, digital-to-analog conversion of compensated signals requires a DAC with high bit precision (i.e., a large number of bits).
However, as an inverse relationship exists between bit precision and the speed of conversion by a DAC, the use of a DAC 216 with high bit precision necessarily involves sacrificing conversion speed to a certain extent. Wireless communications systems developed in recent years, particularly systems using high frequency signals—such as communications systems in CDMA base stations—require DACs that can perform conversion rapidly, and in such systems the sacrifice of conversion speed associated with high bit precision becomes unacceptable.
Another drawback is that a phase difference between the feedback signal and reference signal may be produced in the transmission device due to factors such as delay caused by the length of the transmission path of the feedback signal or phase jitter in the local oscillator that performs band conversion (frequency conversion) of the signal. The presence of a phase difference may make it impossible to perform compensation properly. Further, a DAC having high bit precision may be required in order for the DAC to convert a transmission signal with compensated the phase difference.